cmd/link: increase reserved space for passing env on wasm

On wasm, the wasm_exec.js helper passes the command line arguments and
environment variables via a reserved space in the wasm linear memory.
Increase this reserved space from 4096 to 8192 bytes so more environment
variables can fit into the limit.

Later, after https://golang.org/cl/350737 landed, we can switch to the
WASI interface for getting the arguments and environment. This would
remove the limit entirely.

Fixes #49011

Change-Id: I48a6e952a97d33404ed692c98e9b49c5cd6b269b
Reviewed-on: https://go-review.googlesource.com/c/go/+/358194


Trust: Richard Musiol <neelance@gmail.com>
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

misc/wasm/wasm_exec.js

@@ -519,9 +519,9 @@
 
 			// The linker guarantees global data starts from at least wasmMinDataAddr.
 			// Keep in sync with cmd/link/internal/ld/data.go:wasmMinDataAddr.
-			const wasmMinDataAddr = 4096 + 4096;
+			const wasmMinDataAddr = 4096 + 8192;
 			if (offset >= wasmMinDataAddr) {
-				throw new Error("command line too long");
+				throw new Error("total length of command line and environment variables exceeds limit");
 			}
 
 			this._inst.exports.run(argc, argv);

src/cmd/link/internal/ld/data.go

@@ -2442,10 +2442,11 @@ func splitTextSections(ctxt *Link) bool {
 	return (ctxt.IsPPC64() || (ctxt.IsARM64() && ctxt.IsDarwin())) && ctxt.IsExternal()
 }
 
-// On Wasm, we reserve 4096 bytes for zero page, then 4096 bytes for wasm_exec.js
-// to store command line args. Data sections starts from at least address 8192.
+// On Wasm, we reserve 4096 bytes for zero page, then 8192 bytes for wasm_exec.js
+// to store command line args and environment variables.
+// Data sections starts from at least address 12288.
 // Keep in sync with wasm_exec.js.
-const wasmMinDataAddr = 4096 + 4096
+const wasmMinDataAddr = 4096 + 8192
 
 // address assigns virtual addresses to all segments and sections and
 // returns all segments in file order.